Description
If you write a buffer to a TransformStream's WritableStream with await writer.write() and then modify the buffer after the promise completes, the output stream will be corrupted.
As best as I can tell from the spec, it should be safe to modify the chunk after the promise resolves:
If chunk is mutable, producers are advised to avoid mutating it after passing it to write(), until after the promise returned by write() settles. This ensures that the underlying sink receives and processes the same value that was passed in.
As it stands currently, the only safe way to do it is to slice() the buffer to make a copy of it when you pass it to write().
This was discovered as part of working on a worker that uses wasm to compress the response streams in a passthrough transform stream. The resulting stream would be corrupted with chunks being overwritten with later data.